An Introduction to CGI - The Common Gateway Interface

HTTP Headers

You can send the client any type of output you want, be it an html page, a graphic, a sound clip, or even a reference to another web document. However, before you print your output to the standard output stream, you first need to send an HTTP Header to the client so it knows what type of information you are sending. This will allow the client's software to display the information correctly.

The HTTP server will send most of the HTTP headers required to complete the response to the client, but you should provide the last header which describes the type of information, whether it is an actual document or a reference to one. Usually, you will send an HTTP Content-type header; if you are going to print an html document to stdout, the content-type header should have a value of text/html. If you are sending a plain text document, the value should be text/plain. Here's what the header will look like:
Content-type: text/html\n\n
There are two important things to note here:

1. The header is case sensitive.
2. The header is followed by two newline characters (\n\n).

There are more header types than just "Content-type". For example, if you want to redirect the client to another URL rather than send output, you would send a header called a Location header. It would look something like this:
Location: http://www.JayEckles.Com\n\n
Once again notice the two newlines. Even if you don't send any output, you must send a header followed by two newline characters; else the user will simply be staring at an inactive browser for an indefinite amount of time. For a more detailed explanation of what HTTP headers are available for you to use, refer to the HTTP specification or an HTTP quick-reference. However, in the vast majority of cases you will simply be sending an html document to the user, preceding it with the header "Content-type: text/html\n\n" or sending the user to another location using the header "Location: http://domain.com\n\n". The next most common header is "Content-type: image/gif\n\n" or "Content-type: image/jpg\n\n", followed by an image being printed to stdout.

After you send your HTTP header(s) back to the client, you should then print your output to the client. If you want to send back an html document, either read an existing html document into memory and then print the buffer it is stored in on stdout or just print html tags straight to stdout, thus generating your html page dynamically and on-the-fly. Many people alter the information presented in the html document they return based on the input received by the program.

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